1. Technical Field
The present invention relates to undersea dredging and backfilling and more particularly relates to a method and apparatus for the prevention of underscour and erosion, and backfilling of void spaces around undersea rigs, barges, and rig foundations and the like.
2. General Background
There are numerous types of offshore platforms and rigs which are used for the drilling and/or production of oil and gas wells. Most offshore platforms use a substructure which is somehow based upon the seabottom. Permanent structures use piling which are driven into the underlying soil mass of the seabed a large distance so as to develop sufficient structural integrity so that a jacket or rig structure can be fastened or otherwise affixed to the piling, usually by welding.
Some offshore platforms are of a movable type which can be transported to and from job sites and reused in various places as work occurs. For example, jackup rigs use a plurality of legs which raise and lower a barge above the surface of the water, the legs providing a firm base on the seabed. Another type of offshore platform or rig is a submersible barge supported rig often called a workover rig. These devices use a substructural barge upon which is mounted a jacket or other network of legs or pipes that extend upwardly and support a second superstructure above the water surface. For example, if the workover rig is operating in water depths of fifty feet, the substructural barge might sit on the bottom and the legs of the rig might extend eighty feet upwardly to the deck of the barge which would then be thirty feet above the water surface. It should be understood that the method and apparatus of the present invention is not limited to shallow depths of, e.g., fifty feet. This method of erosion prevention is not limited to any depth, and may be used for deep-draft drilling platforms with piling legs as well as submerged barge types.
Such rigs using a substructural barge do not rely upon piling for anchoring the rig to the seabed. Rather, the substructural barge rests directly upon the seabed itself.
Over a long period of time, undersea currents and the surface currents generated by workboats and other such surface vessels can wash away the seabed or underlying soil mass which supports the substructural barge. In these cases, the barge is unsupported at the washed out area and severe washout can eventually threaten the structural integrity of the entire rig. One solution to this problem is to manually install sandbags which are transported to the work site with a workboat, barge or the like. The sandbags are then manually placed in the void spaces around the rig substructure by divers. This is a very time-consuming and expensive process. Not only must the divers expend a tremendous amount of time and effort to transport small sandbags one at a time to the area which is washed out, but the bags themselves must be first loaded onto a barge and then transported to the site for use. If not enough bags are available, multiple trips must be made. Oftentimes, the owner of the workover rig or other such vessel must actually purchase the sandbags if sand is not readily available or beaches near the rig. The usual case requires purchase of sand in bags such as one hundred pound sacks of sand. These bags are conveyed to the job by barge or workboat. Individual bags are then dumped into the water and properly positioned by divers.